Method and apparatus for modifying depth values using pixel programs

ABSTRACT

A method and apparatus for generating depth values in a programmable graphics system. Depth values are calculated under control of a pixel program using a variety of sources as inputs to programmable computation units (PCUs) in the programmable graphics systems. The PCUs are used to compute traditional interpolated depth values and modified depth values. Th PCUs are also used to compute arbitrary depth values which, unlike traditional interpolated depth values and modified depth values, are not dependent on the coordinates of the geometry primitive with which the arbitrary depth values are associated. Several sources are available as inputs to the PCUs. Clipping with optional clamping is performed using either interpolated depth values or calculated depth values, where calculated depth values are arbitrary depth values or modified depth values. Final depth values, used for depth testing, are selected from interpolated depth values and arbitrary depth values after clipping is performed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from commonly owned U.S. patentapplication No. 60/397,468 entitled “Method and Apparatus for ModifyingDepth Values Using Pixel Programs” filed Jul. 19, 2002 that isincorporated herein by reference.

BACKGROUND

1. Field of the Invention

The invention is in the field of computer graphics, and moreparticularly to calculating depth values in a graphics pipeline.

2. Description of the Related Art

Recent advances in graphics processors permit users to program graphicspipeline units using fragment or pixel programs to implement a varietyof user defined shading algorithms. Traditionally, a graphics processorcalculated an interpolated depth value for each pixel dependent on x andy coordinates of a geometry primitive, e.g., triangle or polygon thatthe interpolated depth value was associated with. The interpolated depthvalue was passed through the graphics pipeline, pixels were clipped bydiscarding the pixels whose interpolated depth value was outside of thespace between the near and far clipping planes, and the retained pixeldepth values were used during the depth test.

A desire for greater flexibility in computations has resulted inmodifications in the traditional graphics processor to permit thecalculation not only of the traditional interpolated depth values, butalso to perform further calculations based on the interpolated depthvalues to generate modified depth values. As illustrated in FIG. 1, aGraphics Subsystem 120 calculates traditional interpolated depth valuesin a Rasterizer 150. The interpolated depth values are pipelined througha Texturer 155 to a Pixel Unit 160. The interpolated depth values areused to compute modified depth values in Pixel Unit 160. The modifieddepth values are received by a Raster Operation Unit 165 that clips thepixels associated with the depth values and performs the depth testoperation.

At least one application programming interface (API), OpenGL® API 1.3,prefers that clipping be performed using interpolated depth valuesrather than modified depth values, even when modified depth values aregenerated. However, when Pixel Unit 160 outputs modified depth values itis not possible for a unit downstream in the pipeline, such as theRaster Operation Unit 165, to clip pixels using the interpolated depthvalues. It is also not possible to recalculate the interpolated depthvalues because the inputs needed for the interpolation calculation arenot available to Raster Operation Unit 165. However, some pixel programsdo clip pixels using the modified depth values. Therefore it is stilldesirable to compute modified depth values and then clip using eithermodified depth values or interpolated depth values.

Further, the interpolated depth values and the modified depth values areboth typically computed dependent on the coordinates of the geometryprimitive. However, there is a desire to implement pixel programs thatcompute depth values that are independent of the coordinates of thegeometry primitive, such as depth values read from memory using mapindices u and v.

For the foregoing reasons, there is a need for a graphics system thatcan generate not only interpolated depth values and modified depthvalues, but also arbitrary depth values that are independent of thecoordinates of a geometry primitive. Furthermore, there is a need for agraphics system that can clip pixels using either calculated depthvalues or interpolated depth values, where calculated depth values areeither modified depth values or arbitrary values, and select betweeninterpolated depth values and calculated depth values to determine thefinal depth values for depth testing.

SUMMARY

The present invention is directed to an system and method that satisfiesthe need for a programmable graphics system that can generate thecalculated depth values and the interpolated depth values, clip andoptionally clamp pixel values using either the calculated pixel depthvalues or interpolated pixel depth values, and select betweeninterpolated pixel depth values and calculated pixel depth values togenerate final pixel depth values for depth testing.

Various embodiments of the invention include a programmable fragmentprocessing pipeline comprising a local register file and a programmablecomputation unit. The programmable computation unit comprises one ormore arithmetic units, under control of a pixel program, and a writeinterface. The programmable computation unit is configured to select,under the control of a pixel program, one or more inputs to the one ormore arithmetic units from a plurality of sources. The one or morearithmetic units use the selected one or more inputs to compute thecalculated pixel depth value that is associated with a geometryprimitive pixel and independent of the coordinates of the geometryprimitive. The calculated pixel depth value is optionally written to thelocal register file. Additionally, the invention can include a hostprocessor, a host memory, and a system interface configured to interfacewith the host processor.

Furthermore, the one or more arithmetic units are configured to computethe interpolated pixel depth value associated with a geometry primitivepixel and dependent on the coordinates of the geometry primitive. Stillfurther, an interpolated pixel depth value is computed using aprogrammable depth computation unit. A selector is configured, under thecontrol of a pixel program, to select either the interpolated pixeldepth value computed by the programmable depth computation unit or thecalculated pixel depth value as the final pixel depth value. Theplurality of sources includes a graphics memory, a host memory, and another programmable computation unit in the programmable fragmentprocessing pipeline.

An embodiment of the programmable fragment processing pipeline includesa first clipping unit configured to clip and optionally clamp a pixelassociated with the interpolated pixel depth value. The programmablefragment processing pipeline also includes a second clipping unitconfigured to clip and optionally clamp a pixel associated with thefinal pixel depth value.

An embodiment of the present invention includes a method of calculatinga pixel depth value comprising selecting inputs for a depth calculationfrom a plurality of sources, computing a calculated pixel depth value,and writing the calculated pixel depth value to a local register file,where the selecting, computing, and writing are performed under controlof a pixel program. The calculated pixel depth value is associated witha geometry primitive pixel and independent of the coordinates of thegeometry primitive. Additionally, the method can use a computing systemincluding a programmable fragment processing pipeline to calculate thepixel depth value.

Furthermore, the method can include calculation of a first interpolatedpixel depth value that is associated with a geometry primitive pixel anddependent on the coordinates of the geometry primitive. The method caninclude selection, under control of the pixel program, between the firstinterpolated pixel depth value and calculated pixel depth value,generating a final pixel depth value. The method can further includecomputing, under control of the pixel program, a second interpolatedpixel depth value that is the same as the first interpolated pixel depthvalue. The plurality of sources includes a graphics memory, a hostmemory, and an other programmable computation unit in the programmablefragment processing pipeline.

Still further, the method can include clipping and optionally clamping apixel associated with the interpolated pixel depth value. Yet further,the method can include clipping and optionally clamping a pixelassociated with the final pixel depth value.

BRIEF DESCRIPTION OF THE VARIOUS VIEWS OF THE DRAWINGS

Accompanying drawing(s) show exemplary embodiment(s) in accordance withone or more aspects of the present invention; however, the accompanyingdrawing(s) should not be taken to limit the present invention to theembodiment(s) shown, but are for explanation and understanding only.

FIG. 1 is a block diagram illustrating a prior art general computingsystem including a graphics subsystem;

FIG. 2 illustrates one embodiment of a computing system according to theinvention including a host computer and a graphics subsystem;

FIG. 3 is a block diagram of an embodiment of the Shader of FIG. 2;

FIG. 4 is a block diagram of the units which process depth in anembodiment of the Shader Core of FIG. 3; and

FIG. 5 is a flowchart illustrating the processing of pixel programs bythe units shown FIG. 4.

DESCRIPTION

The current invention involves new systems and methods for processingand generating depth values in a programmable shader pipeline. Thesesystems and methods satisfy the need for a graphics system that cangenerate the calculated depth values and the interpolated depth values,clip pixels using either the calculated depth values or interpolateddepth values, and select between interpolated depth values andcalculated depth values to determine final depth values for depthtesting.

FIG. 2 is an illustration of a Computing System generally designated 200and including a Host Computer 110 and a Graphics Subsystem 210.Computing System 200 may be a desktop computer, server, laptop computer,palm-sized computer, tablet computer, game console, cellular telephone,computer based simulator, or the like. Host Computer 110 includes HostProcessor 114 which may include a system memory controller to interfacedirectly to Host Memory 112 or may communicate with Host Memory 112through a System Interface 115. System Interface 115 may be an I/O(input/output) interface or a bridge device including the system memorycontroller to interface directly to Host Memory 112. Host Computer 110communicates with Graphics Subsystem 210 via System Interface 115 and anInterface 217. Data received at Interface 217 can be passed to aGeometry Processor 230 or written to a Local Memory 240 through MemoryController 220. Memory Controller 220 is configured to handle data sizesfrom typically 8 to more than 128 bits.

A Graphics Processing Pipeline 205 includes, among other components,Geometry Processor 230 and a Fragment Processing Pipeline 260 that eachcontain one or more programmable graphics processing units to perform avariety of specialized functions. Some of these functions are tablelookup, scalar and vector addition, multiplication, division,coordinate-system mapping, calculation of vector normals, tessellation,calculation of derivatives, interpolation, and the like. GeometryProcessor 230 and Fragment Processing Pipeline 260 are optionallyconfigured such that data processing operations are performed inmultiple passes through Graphics Processing Pipeline 205 or in multiplepasses through Fragment Processing Pipeline 260. Shader programs andpixel programs are sequences of program instructions compiled forexecution within Fragment Processing Pipeline 260. Data generated undercontrol of a program in one pass and written to Local Memory 240 or HostMemory 112, e.g., a texture map, can be accessed in a subsequent pass.Alternatively, the data written to Local Memory 240 or Host Memory 112can be optionally processed and used as indices to access map data, suchas a texture map, stored in Local Memory 240 or Host Memory 112.Furthermore, a Shader 255, within Fragment Processing Pipeline 260, isoptionally configured using the pixel programs such that data processingoperations are performed in multiple passes within Shader 255. It shouldbe understood that the program instructions and data can be read from orwritten to memory, e.g., any combination of Local Memory 240 and HostMemory 112.

Geometry Processor 230 receives a stream of program instructions anddata and performs vector floating-point operations or other processingoperations. Processed data is passed from Geometry Processor 230 to aRasterizer 250. In a typical implementation Rasterizer 250 performs scanconversion and outputs fragment, pixel, or sample data and programinstructions to Fragment Processing Pipeline 260. For simplicity, theremainder of this description will use the term pixels to refer topixels, samples and/or fragments.

Data processed by Shader 255 is passed to a Raster Analyzer 265, whichperforms operations similar to prior art Raster Operation Unit 165 andsaves the results in Local Memory 240 or a Host Memory 112. RasterAnalyzer 265 includes a read interface and a write interface to MemoryController 220 through which Raster Analyzer 265 accesses data,including pixel depth values, stored in Local Memory 240 or Host Memory112.

When processing is completed, an Output 285 of Graphics Subsystem 210 isprovided using an Output Controller 280. Output Controller 280 isoptionally configured to deliver data to a display device, network,electronic control system, other Computing System 200, other GraphicsSubsystem 210, or the like.

FIG. 3 is a block diagram of Fragment Processing Pipeline 260 includingprogrammable graphics processing units Shader 255 and Raster Analyzer265. Shader 255 and Raster Analyzer 265 use program instructions toprocess graphics data as described further herein. The programinstructions and graphics data are stored in graphics memory, e.g.,Local Memory 240 and storage resources within Fragment ProcessingPipeline 260 such as register files, and the like.

A Shader Triangle Unit 310 calculates the plane equations for texturecoordinates, depth, and other parameters. A Gate Keeper 320 performs amultiplexing function, selecting between the pipeline data fromRasterizer 250 and Shader Triangle Unit 310 and a Feedback Output 376 ofa Combiners 370. Shader Core 330 initiates Local Memory 240 readrequests via Texture 340 that are processed by Memory Controller 220 toread data, such as map data (e.g., height field, bump, texture, etc.)and program instructions. Shader Core 330 also performs floating pointcomputations such as triangle parameter interpolation and reciprocalsand is optionally programmed to compute the interpolated pixel depthvalues. Pixel data processed by Shader Core 330 is optionally input to aCore Back End FIFO (first in first out) 390. Even when interpolatedpixel depth values are computed in Shader Core 330, interpolated pixeldepth values are not necessarily output to Core Back End FIFO 390.Instead plane equation data needed to compute interpolated pixel depthvalues is output to Core Back End FIFO 390 and interpolated pixel depthvalues are recomputed by a Shader Back End 360, as described furtherherein, because plane equation data requires fewer pipeline storageresources than interpolated pixel depth values. Futhermore, in analternate embodiment Core Back End FIFO 390 is implemented as a registerfile that is written by Shader Core 330 and read by Shader Back End 360.

Data read by Shader Core 330 via Memory Controller 220 is returned to aTexture 340. Texture 340 unpacks the read data and outputs unpacked datato a Remap 350. Remap 350 interprets any program instructions includedin the unpacked data and generates codewords which control theprocessing completed by the graphics processing units in FragmentProcessing Pipeline 260. When multi-pass operations are being performedwithin Shader 255, Remap 350 also reads the data fed back from Combiners370 via a Quad Loop Back 356, synchronizing the fed back data with theunpacked data received from Texture 340, as explained more fully herein.Remap 350 formats the unpacked data and fed back data, outputtingcodewords and formatted data to Shader Back End 360.

Shader Back End 360 also receives pixel component data from Shader Core330 via Core Back End FIFO 390 and triangle data, such as planeequations, from Gate Keeper 320. Shader Back End 360 synchronizes pixelcomponent data and triangle data with formatted data from Remap 350.Shader Back End 360 performs computations using the input data(formatted data, pixel component data and triangle data) based oncodewords received from Remap 350. Specifically, Shader Back End 360computes interpolated pixel depth values using plane equation data fromShader Triangle Unit 310 via Gate Keeper 320 and pixel component datawritten to Core Back End FIFO 390 by Shader Core 330. Those computedinterpolated pixel depth values computed in Shader Back End 360 are thesame, pixel for pixel, as interpolated pixel depth values optionallycomputed in Shader Core 330. Therefore, pipeline storage resourcesrequired to pass interpolated pixel depth values from Shader Core 330 toShader Back End 360 are not needed and the interpolated pixel depthvalues are as accurate as those computed in Shader Core 330. In ShaderBack End 360, interpolated pixel depth values are used to clip the pixelvalues associated with each interpolated depth value. The clipped pixelvalues are optionally clamped to valid x and y coordinates to avoiddiscarding a partially clipped pixel that is slightly outside of theclip space due to the precision of the interpolation computation. ShaderBack End 360 optionally computes calculated pixel depth values usingformatted data containing read map data and/or read fed back data.Finally, Shader Back End 360 uses a multiplexor to select betweeninterpolated pixel depth values and calculated pixel depth values,choosing final pixel depth values for output as part of a stream alsoincluding codewords and shaded pixel data.

The output of Shader Back End 360 is input to Combiners 370 wherecodewords are executed by the programmable combiner computation unitswithin Combiners 370. Combiners 370 are typically used to performarithmetic computations using two or more inputs received from ShaderBack End 360 to generate combined pixel data. Arithmetic computationsinclude dot products, multiplication, and addition, among others. Thecodewords executing in the current pass control whether the combinedpixel data will be fed back within Shader 255, using one or both of thepaths, to be processed in a subsequent pass. Using a first path,Combiners 370 optionally output codewords, to be executed by Shader Core330 and Texture 340 in a subsequent pass, to Gate Keeper 320 usingfeedback path 376. Using a second path, Combiners 370 also optionallyoutput combined pixel data to local register file Quad Loop Back 356, tobe read by Remap 350 in a subsequent pass. Finally, Combiners 370optionally output combined pixel data, e.g., x, y, color, depth, otherparameters, to Raster Analyzer 265. Raster Analyzer 265 performs nearand far plane clipping and raster operations, such as stencil, z test,etc., using the combined pixel data and pixel data stored in LocalMemory 240 or Host Memory 112 at the x,y location associated with thecombined pixel data. The output data from Raster Analyzer 265 is writtenback to Local Memory 240 or Host Memory 112 via Memory Controller 220 atthe x,y locations associated with the output data. The output data isrepresented in one or more formats as specified by the codewords. Forexample, color data is written as 16 or 32 bit per pixel ARGB to bescanned out for display or used as a texture map by a pixel programexecuted in a subsequent pass within Fragment Processing Pipeline 260 orthrough Graphics Processing Pipeline 205. Alternatively, color and depthdata is written, and later read and processed by Raster Analyzer 265 togenerate output data prior to being scanned out for display via OutputController 280.

FIG. 4 is a block diagram of the elements of Shader 255 and RasterAnalyzer 265 that are used to generate or process pixel depth values.Therefore, the block diagram is only a partial representation of Shader255 and Raster Analyzer 265. The functions of the different elements areexplained with reference to the flowchart of FIG. 5.

FIG. 5 is a flowchart representing one method of the invention forgenerating or processing depth values using the programmable graphicsshader of the invention. The interpolated depth value computed using thegeometric data for a primitive, e.g., triangle, is calculated in Shader255 by following the sequence of steps in Example 1 described below. InExample 1, the final depth value, optionally written back to LocalMemory 240 via Memory Controller 220, is interpolated depth.

EXAMPLE 1

In step 510, Gate Keeper 320 receives pixel data, triangle data, andcodewords from Rasterizer 250 and Shader Triangle Unit 310. Gate Keeper320 stores triangle data, including depth plane equation data, inTriangle Memory 410 and outputs pixel data and codewords to Shader Core330 via Multiplexor 415. In step 512, Shader Core 330 configures theProgrammable Computation Unit (PCU), PCU1 420, according to thecodewords, to generate pixel output data based on data received fromGate Keeper 320. Per pixel interpolated depth values are optionallycomputed using PCU1 420 to evaluate plane equations. In an alternateembodiment one or more additional PCUs are included in Shader Core 330such that pixel texture coordinates, pixel parameter values, per pixelinterpolated depth values, and the like, are computed in parallel. PCU1420 includes arithmetic subunits, logic for selection of inputs to thearithmetic subunits, and interface logic to write register files orFIFOs. The interface logic generates the write address and write controlsignals based on the protocol required by a register file or FIFO.

In step 514, Shader Core 330 writes per pixel components to Core BackEnd FIFO 390. Data stored in Core Back End FIFO 390 is used as sourcedata in the current pass through Shader 255 or alternatively, in asubsequent pass. Per pixel interpolated depth values computed by PCU1420 are effectively discarded because per pixel interpolated depthvalues are not written to Core Back End FIFO 390. In step 516, ShaderCore 330 uses the codewords to determine whether map data or programinstructions to be executed at a later time are required to be read fromLocal Memory 240. The codewords are passed from Shader Core 330 throughTexture 340 to Remap 350. If the program instructions are required to beread from Local Memory 240, in step 530, Shader Core 330 calculates theread addresses in Address Generator 425 and outputs a read request toMemory Controller 220. In step 534, read program instructions return toTexture 340 and are unpacked. In step 536, Remap 350 receives andprocesses the unpacked program instructions, generates codewords, anddetermines whether source data is required to be read from Quad LoopBack 356, and, if not, in step 542, Remap 350 outputs codewords toShader Back End 360. In this example, in step 544, codewords received byShader Back End 360 configure programmable computation unit, PCU2 430,to be idle rather than perform a computation because there is no sourceinput data for PCU2 430 to process. Then, in step 518 Shader Back End360 uses the codewords to determine whether this is the last pass of thedata through Shader 255 and, if so, in step 520, Shader Back End 360reads per pixel components from Core Back End FIFO 390 and depth planeequation data from Triangle Memory 410 to compute interpolated pixeldepth values using a Depth Processing Unit (DPU) 450. Interpolated pixeldepth values are used by a Clip 455 to clip pixels using near and farclipping planes. Clipped pixel values are optionally clamped to valid xand y coordinates to avoid discarding a partially clipped pixel that isslightly outside of the clip space due to the precision of theinterpolation computation. Similar to the PCU1 420, DPU 450functionality is not limited to interpolation computations.

In step 522, Shader Back End 360 determines if “depth replace” isenabled according to the codewords and, if not, in step 546, Multiplexor465 selects DPU 450 computed interpolated and clipped pixel depth valuesas final pixel depth values that are output from Shader Back End 360. Inan alternate embodiment the selection function is performed usingcombinatorial logic, a lookup table, or the like. In step 524, Combiners370 uses the codewords to determine whether this is the last pass of thedata through Shader 255 and, if so, in step 526, Combiners 370 inputfinal pixel depth values into a Depth FIFO 475 that are later output toRaster Analyzer 265. In step 528, Raster Analyzer 265 receives finalpixel depth values from Combiners 370 and performs near and far planeclipping with optional clamping. Raster Analyzer 265 optionally reads,via Memory Controller 220, pixel depth values stored in Local Memory 240corresponding to pixel (x, y) locations for final pixel depth values.Raster Analyzer 265 optionally performs a depth test function using readpixel depth values and final pixel depth values as specified by thecodewords and generates a pass or fail result. If the depth test passes,final depth is written back to Local Memory 240 via Memory Controller220. If the depth test fails the final depth is discarded. In thisexample, final depth is clipped interpolated pixel depth.

In Example 2, a calculated depth value is computed and used as finalpixel depth in Shader 255 by following the sequence of steps describedbelow. Source data used to calculate pixel depth values is stored inLocal Memory 240 as a map and is accessed using u and v indicesassociated with specific geometric locations. In this example, the mapdata is an array of depth values that are processed in the same manneras texture map data is processed, e.g., trilinearly interpolated. Themap data is independent of the vertex coordinates x, y, and z.Therefore, unlike final pixel depth values in Example 1, final pixeldepth values in Example 2 are independent of the coordinates of thegeometry primitive.

EXAMPLE 2

In step 510, Gate Keeper 320 receives pixel data, triangle data, andcodewords from Rasterizer 250 and Shader Triangle Unit 310. Gate Keeper320 stores triangle data, including depth plane equation data, inTriangle Memory 410 and outputs pixel data and codewords to Shader Core330 via Multiplexor 415. In step 512, Shader Core 330 configures PCU1420 according to the codewords to generate pixel output data based ondata received from Gate Keeper 320. Per pixel interpolated depth valuesare optionally computed using PCU1 420 to evaluate plane equations. Instep 514, Shader Core 330 writes per pixel components to Core Back EndFIFO 390.

In step 516, Shader Core 330 uses the codewords to determine whether mapdata or program instructions to be executed at a later time are requiredto be read from local memory. The codewords are passed from Shader Core330 through Texture 340 to Remap 350. If map data or programinstructions to be executed at a later time are required to be read fromLocal Memory 240, in step 530, Shader Core 330 calculates read addressesin Address Generator 425 and outputs read requests to Memory Controller220 via Texture 340. In step 534, read depth map data values and readprogram instructions return to Texture 340 and are unpacked. In step536, Remap 350 receives and processes the unpacked program instructions,generates codewords, and determines whether source data is required tobe read from Quad Loop Back 356. If the source data stored in Quad LoopBack 356 is not required to be read, in step 542 Remap 350 formatconverts unpacked depth map data received from Texture 340 and outputsformat converted depth data and codewords to Shader Back End 360. Instep 544, codewords received by Shader Back End 360 configure PCU2 430to perform trilinear interpolation using the format converted depthdata. In step 518, Shader Back End 360 uses the codewords to determinewhether this is the last pass of the data through Shader 255, and, ifso, in Step 520 Shader Back End 360 reads per pixel components from CoreBack End FIFO 390 and triangle data from Triangle Memory 410 andcomputes interpolated pixel depth values using DPU 450. Interpolatedpixel depth values are used by Clip 455 to clip pixels using near andfar clipping planes and optionally clamp clipped pixel values to avoiddiscarding a partially clipped pixel.

In step 522, Shader Back End 360 determines if “depth replace” isenabled according to the codewords and, if so, in step 548 Multiplexor465 selects PCU2 430 trilinearly interpolated pixel depth values asfinal pixel depth values to be output from Shader Back End 360 toCombiners 370. In step 524, Combiners 370 uses the codewords todetermine whether this is the last pass of the data through Shader 255and, if so, in step 526 Combiners 370 input final pixel depth valuesinto a Depth FIFO 475 that are later output to Raster Analyzer 265. Instep 528, Raster Analyzer 265 receives final pixel depth values fromCombiners 370 and performs near and far plane clipping with optionalclamping. Raster Analyzer 265 optionally reads pixel depth values storedin Local Memory 240 corresponding to pixel (x, y) locations for finalpixel depth values. Raster Analyzer 265 optionally performs a depth testfunction using read pixel depth values and final pixel depth values asspecified by the codewords and generates a pass or fail result. If thedepth test passes, final depth is written back to Local Memory 240 viaMemory Controller 220. If the depth test fails the final depth isdiscarded. In this example, final depth is PCU2 430 calculated pixeldepth that was generated independent from the corresponding interpolateddepth value computed using the coordinates of the geometry primitive.

In Example 3, two calculated pixel depth values are computed andcombined to output a new depth value in Shader 255 by following thesequence of steps described below. As a result of the independent pathsand programmable configuration of Shader 255, programming Shader 255 inthe configuration in this combination permits displacements read from amap stored in Local Memory 240 to be applied to interpolated pixel depthvalues calculated in Shader Core 320.

EXAMPLE 3

In step 510, Gate Keeper 320 receives pixel data, triangle data, andcodewords from Rasterizer 250 and Shader Triangle Unit 310. Gate Keeper320 stores triangle data, including depth plane equation data, inTriangle Memory 410 and outputs pixel data and codewords to Shader Core330. In step 512, Shader Core 330 configures PCU1 420 according to thecodewords to generate pixel output data based on data received from GateKeeper 320. Per pixel interpolated depth values are optionally computedusing PCU1 420 to evaluate plane equations. In step 514, Shader Core 330writes per pixel interpolated depth values to Core Back End FIFO 390 tobe used as source data in the current pass through Shader 255.

In step 516, Shader Core 330 uses codewords to determine whether mapdata or program instructions to be executed at a later time are requiredto be read from Local Memory 240. The codewords are passed from ShaderCore 330 through Texture 340 to Remap 350. If the map data or programinstructions to be executed at a later time are required to be read fromLocal Memory 240, in step 530 Shader Core 330 calculates the readaddresses in Address Generator 425 and outputs read requests to MemoryController 220. In step 534, read depth map data values and read programinstructions return to Texture 340 and are unpacked. In step 536, Remap350 receives and processes the unpacked program instructions, generatescodewords, and determines whether source data is required to be readfrom Quad Loop Back 356. If the source data stored in Quad Loop Back 356is not required to be read, in step 542 Remap 350 format convertsunpacked depth map data received from Texture 340. The format converteddepth data and codewords are output by Remap 350 to Shader Back End 360.In step 544, Shader Back End 360 configures PCU2 430 according to thecodewords to perform a computation. Interpolated pixel depth valuescalculated using PCU1 420 in Shader Core 330 and stored in Core Back EndFIFO 390 are also input to Shader Back End 360 and both depth values areprocessed by PCU2 430. In this example, PCU2 430 is configured to useformat converted depth values as displacements and modify interpolatedpixel depth values to compute displaced pixel depth values.Alternatively, PCU1 420 computed interpolated pixel depth values arecombined with format converted depth values using PCU2 430.

In step 518, Shader Back End 360 uses the codewords to determine whetherthis is the last pass of the data through Shader 255, and, if so, inStep 520 Shader Back End 360 reads per pixel components from Core BackEnd FIFO 390 and triangle data from Triangle Memory 410 and computesinterpolated pixel depth values using DPU 450. Interpolated pixel depthvalues are used by Clip 455 to clip pixels using near and far clippingplanes and optionally clamp clipped pixel values to avoid discarding apartially clipped pixel.

In step 522, Shader Back End 360 determines if “depth replace” isenabled according to the codewords and, if so, in step 548 Multiplexor465 selects PCU2 430 calculated displaced pixel depth values as finalpixel depth values to be output from Shader Back End 360 to Combiners370. In step 524, Combiners 370 uses the codewords to determine whetherthis is the last pass of the data through Shader 255 and, if so, in step526 Combiners 370 input final pixel depth values into Depth FIFO 475that are later output to Raster Analyzer 265. In step 528, RasterAnalyzer 265 receives final pixel depth values from Combiners 370 andperforms near and far plane clipping with optional clamping. RasterAnalyzer 265 optionally reads pixel depth values stored in Local Memory240 corresponding to the pixel (x, y) locations for final pixel depthvalues. Raster Analyzer 265 optionally performs a depth test functionusing read pixel depth values and final pixel depth values as specifiedby the codewords and generates a pass or fail result. If the depth testpasses, final depth is written back to Local Memory via MemoryController. If the depth test fails the final depth is discarded. Inthis example, final depth is displaced depth that was generated usinginterpolated pixel depth values computed from the coordinates of thegeometry primitive and map data representing depth displacements.

In Example 4 pixel depth values are computed in two passes throughShader 255 to output a new pixel depth value following the sequence ofsteps described below. Programming Shader 255 in the configuration inthis combination permits values computed during a first pass to be usedto calculate depth values in Shader Back End 360 during a second pass.In this example, depth displacements are computed during the first passand the displacements are applied to interpolated pixel depth valuesduring the second pass.

EXAMPLE 4

In step 510, Gate Keeper 320 receives pixel data, triangle data, andcodewords from Rasterizer 250 and Shader Triangle Unit 310. Gate Keeper320 stores triangle data, including depth plane equation data, inTriangle Memory 410 and outputs pixel data and codewords to Shader Core330. In step 512, Shader Core 330 configures PCU1 420 according tocodewords and computes depth displacements based on data received fromGate Keeper 320. In step 514, Shader Core 330 writes per pixel depthdisplacements to Core Back End FIFO 390 to be used as source data in thesecond pass through Shader 255.

In step 516, Shader Core 330 uses codewords to determine whether mapdata or program instructions to be executed at a later time are requiredto be read from Local Memory 240. The codewords are passed from ShaderCore 330 through Texture 340 to Remap 350. If the map data or programinstructions to be executed at a later time are not required to be readfrom Local Memory 240, in step 538 Remap 350 determines whether sourcedata is required to be read from Quad Loop Back 356. If the map sourcedata is not required to be read from Quad Loop Back 356 Remap 350outputs codewords to Shader Back End 360. In step 544, Shader Back End360 configures PCU2 430 according to codewords to pass the data input toPCU2 430 through to the output of PCU2 430. In this example per pixeldepth displacements calculated using PCU1 420 in Shader Core 330 andstored in Core Back End FIFO 390 are passed through PCU2 430.Alternatively, PCU2 430 is configured to compute modified pixel depthdisplacements using source inputs such as read map data or pixelcomponents read from Core Back End FIFO 390.

In step 518, Shader Back End 360 uses the codewords to determine whetherthis is the last pass of the data through Shader 255 and, if not, instep 522 Shader Back End 360 determines if “depth replace” is enabledaccording to the codewords. If “depth replace” is enabled, in step 548Multiplexor 465 selects PCU2 430 calculated data and outputs it toCombiners 370. In step 524, Combiners 370 uses the codewords todetermine whether this is the last pass of the data through Shader 255,and, if not, in step 550 Combiners input PCU2 430 processed data outputfrom Shader Back End 360 into combiner computation unit, CCU 470, andfeeds the output of CCU 470 into Gate Keeper 320. Codewords generatedfrom program instructions and data that were each optionally read fromLocal Memory 240 are output by Combiners 370 to Gatekeeper 320.

Example 4 continues with step 510, where Gate Keeper 320 receives CCU470 processed data and synchronizes it with pixel data from Rasterizer250 and triangle data from Shader Triangle Unit 310 using Multiplexor415 to output data received from each source as directed by codewords.In step 512, Shader Core 330 configures PCU1 420 according to codewordsand computes interpolated pixel depth values based on data received fromGate Keeper 320. In step 514, Shader Core 330 writes per pixelinterpolated depth values to Core Back End FIFO 390 to be used as sourcedata in the current pass through Shader 255.

In step 516, Shader Core 330 uses codewords to determine whether mapdata or program instructions to be executed at a later time are requiredto be read from Local Memory 240. The codewords are passed from ShaderCore 330 through Texture 340 to Remap 350. If the map data or programinstructions to be executed at a later time are not required to be readfrom Local Memory 240, in step 538 Remap 350 determines whether sourcedata is required to be read from Quad Loop Back 356. If the source datastored in Quad Loop Back 356 is required to be read, in step 540 Remap350 generates a read request for Quad Loop Back 356. In step 540, depthdisplacements calculated during the first pass are received from QuadLoop Back 356 by Remap 350 and in step 542 Remap 350 format convertsdepth displacements. The format converted depth displacements andcodewords are output by Remap 350 to Shader Back End 360. In step 544,Shader Back End 360 configures PCU2 430 according to codewords toperform a computation. Interpolated pixel depth values calculated usingPCU1 420 in Shader Core 330 and stored in Core Back End FIFO 390 arealso input to Shader Back End 360 and depth displacements are applied tointerpolated pixel depth values using PCU2 430.

In step 518, Shader Back End 360 uses the codewords to determine whetherthis is the last pass of the data through Shader 255 and, if so, in Step520 Shader Back End 360 reads per pixel components from Core Back EndFIFO 390 and triangle data from Triangle Memory 410 and computesinterpolated pixel depth values using DPU 450. Interpolated pixel depthvalues are used by Clip 455 to clip pixels using near and far clippingplanes and optionally clamp clipped pixel values to avoid discarding apartially clipped pixel. In step 522, Shader Back End 360 determines if“depth replace” is enabled according to the codewords and, if so, instep 548 Multiplexor 465 selects PCU2 430 calculated displaced pixeldepth values as final pixel depth values to be output from Shader BackEnd 360 to Combiners 370. In step 524, Combiners 370 uses the codewordsto determine whether this is the last pass of the data through Shader255 and, if so, in step 526 Combiners 370 input final pixel depth valuesinto Depth FIFO 475 that are later output to Raster Analyzer 265. Instep 528, Raster Analyzer 265 receives final pixel depth values fromCombiners 370 and performs near and far plane clipping with optionalclamping. Raster Analyzer 265 optionally reads pixel depth values storedin Local Memory 240 corresponding the pixel (x, y) locations for finalpixel depth values. Raster Analyzer 265 optionally performs a depth testfunction using read pixel depth values and final pixel depth values asspecified by the codewords and generates a pass or fail result. If thedepth test passes, final depth is written back to Local Memory viaMemory Controller. If the depth test fails the final depth is discarded.In this example, final depth is displaced pixel depth values calculatedin two passes where pixel depth displacements are computed during afirst pass and interpolated pixel depth values are computed in a secondpass from the coordinates of the geometry primitive and combined withpixel depth displacements.

In Example 5, depth values are computed in three passes to output a newdepth value in Shader by following the sequence of steps describedbelow. Programming Shader 255 in the configuration detailed in thisexample results in Shader 255 first calculating normal vectors for eachpixel that are used in a second pass to displace interpolated pixeldepth values. In the third pass u, v coordinates are interpolated andused to read data stored in Local Memory 240, e.g., depth displacements.The read map depth displacements are combined with calculated normalvector displaced depth during the third and final pass.

EXAMPLE 5

In step 510, Gate Keeper 320 receives pixel data, triangle data, andcodewords from Rasterizer 250 and Shader Triangle Unit 310. Gate Keeper320 stores triangle data in Triangle Memory 410 and outputs pixel dataand codewords to Shader Core 330. In step 512, Shader Core 330configures PCU1 420 according to the codewords and computes interpolatednormal vectors based on data received from Gate Keeper 320. In step 514,Shader Core 330 writes per pixel normal vectors to Core Back End FIFO390 to be used as source data in the second pass through Shader 255.

In step 516, Shader Core 330 uses the codewords to determine whether mapdata or program instructions to be executed at a later time are requiredto be read from Local Memory 240. The codewords are passed from ShaderCore 330 through Texture 340 to Remap 350. If the program instructionsare required to be read from Local Memory 240, in step 530 Shader Core330 calculates the read addresses in Address Generator 425 and outputs aread request to Memory Controller 220. In step 534, read programinstructions return to Texture 340 and are unpacked. In step 536, Remap350 receives and processes the unpacked program instructions, generatescodewords, and determines whether source data is required to be readfrom Quad Loop Back 356 and, if not, in step 542 Remap 350 outputscodewords to Shader Back End 360. In step 544, Shader Back End 360configures PCU2 430 according to codewords to pass the data input toPCU2 430 through to the output of PCU2 430. In this example, per pixelnormal vectors computed using PCU1 420 in Shader Core 330 and stored inCore Back End FIFO 390 are passed through PCU2 430. Alternatively, PCU2430 is configured to compute modified normal vectors using source inputssuch as read map data or pixel components read from Core Back End FIFO390.

In step 518, Shader Back End 360 uses the codewords to determine whetherthis is the last pass of the data through Shader 255 and, if not, instep 522 Shader Back End 360 determines if “depth replace” is enabledaccording to the codewords. If “depth replace” is enabled, in step 548Multiplexor 465 selects PCU2 430 calculated data and outputs it toCombiners 370. In step 524, Combiners 370 uses the codewords todetermine whether this is the last pass of the data through Shader 255and, if not, in step 550 Combiners inputs PCU2 430 processed data outputfrom Shader Back End 360 into CCU 470 and feeds the output of CCU 470into Gate Keeper 320. Codewords generated from program instructions areoutput by Combiners 370 to Gatekeeper 320. CCU 470 processed data, e.g.,per pixel normal vectors, are written to Quad Loop Back 356 to be usedduring the second pass.

Example 5 continues with step 510 for a second pass, when Gate Keeper320 receives and synchronizes the codewords received from Combiners 370with pixel data from Rasterizer 250 and triangle data from ShaderTriangle Unit 310 using Multiplexor 415 to output data received fromeach source as directed by codewords. In step 512, Shader Core 330configures PCU1 420 according to the codewords and computes interpolatedpixel depth values based on data received by Gate Keeper 320 fromRasterizer 250 and Shader Triangle Unit 310. In step 514, Shader Core330 writes per pixel interpolated depth values to Core Back End FIFO 390to be used as source data in the current pass through Shader 255.

In step 516, Shader Core 330 uses codewords to determine whether mapdata or program instructions to be executed at a later time are requiredto be read from Local Memory 240. The codewords are passed from ShaderCore 330 through Texture 340 to Remap 350. If the map data or programinstructions to be executed at a later time are not required to be readfrom Local Memory 240, in step 538 Remap 350 determines whether sourcedata is required to be read from Quad Loop Back 356. If the source datastored in Quad Loop Back 356 is required to be read, in step 540 Remap350 generates a read request for Quad Loop Back 356. In step 540, pixelnormal vectors calculated during the first pass are received from QuadLoop Back 356 by Remap 350 and in step 542 Remap 350 format convertspixel normal vectors. The format converted pixel normal vectors areoutput by Remap 350 to Shader Back End 360. In step 544, Shader Back End360 configures PCU2 430 according to the codewords to perform acomputation. Interpolated pixel depth values calculated using PCU1 420in Shader Core 330 and stored in Core Back End FIFO 390 are also inputto Shader Back End 360 and PCU2 430 is configured to use formatconverted pixel normal vectors to displace interpolated pixel depthvalues.

In step 518, Shader Back End 360 uses the codewords to determine whetherthis is the last pass of the data through Shader 255 and, if not, instep 522 Shader Back End 360 determines if “depth replace” is enabledaccording to the codewords. If “depth replace” is enabled, in step 548Multiplexor 465 selects PCU2 430 calculated normal vector displaceinterpolated pixel depth values to be output to Combiners 370 beforeproceeding to step 524. In step 524, Combiners 370 uses the codewords todetermine whether this is the last pass of the data through Shader 255and, if not, in step 550 Combiners inputs PCU2 calculated normal vectordisplaced interpolated pixel depth values output from Shader Back End360 into CCU 470 and feeds the output of CCU 470 into Gate Keeper 320.Codewords generated from program instructions are output by Combiners370 to Gatekeeper 320. CCU 470 processed data, e.g., calculated normalvector displace interpolated pixel depth values, are written to QuadLoop Back 356 to be used during the third pass.

Example 5 continues with step 510 for a third pass, when Gate Keeper 320receives and synchronizes the codewords received from Combiners 370 withpixel data from Rasterizer 250 and triangle data from Shader TriangleUnit 310 using Multiplexor 415 to output data received from each sourceas directed by codewords. In step 512, Shader Core 330 configures PCU1420 according to the codewords and computes interpolated map indicesbased on data received by Gate Keeper 320 from Rasterizer 250 and ShaderTriangle Unit 310. In step 514, Shader Core 330 writes per pixelcomponent data to Core Back End FIFO 390 to be used as source data inthe current pass through Shader 255.

In step 516, Shader Core 330 uses the codewords to determine whether mapdata or program instructions to be executed at a later time are requiredto be read from local memory. The codewords are passed from Shader Core330 through Texture 340 to Remap 350. If the map data or programinstructions to be executed at a later time are required to be read fromLocal Memory 240, in step 530 Shader Core 330 calculates the readaddresses in Address Generator 425 and outputs read requests to MemoryController 220. In step 534, read depth map data values and read programinstructions return to Texture 340 and are unpacked. In step 534, Remap350 receives and processes the unpacked program instructions and in step536 determines whether source data are required to be read from QuadLoop Back 356. If the source data stored in Quad Loop Back 356 isrequired to be read, in step 540 Remap 350 generates a read request forQuad Loop Back 356. In step 540, normal vector displaced interpolatedpixel depth values, calculated during the second pass, are received fromQuad Loop Back 356 by Remap 350. In step 542, Remap 350 format convertsread depth displacements and displaced interpolated pixel depth valuesthat were calculated during the second pass. Remap 350 outputscodewords, format converted read depth displacements, and formatconverted displaced interpolated pixel depth values. In step 544, PCU2430 is configured to use format converted read depth displacements tofurther displace format converted displaced interpolated pixel depthvalues and generate displaced pixel depth values.

In step 518, Shader Back End 360 uses the codewords to determine whetherthis is the last pass of the data through Shader 255 and, if so, in Step520 Shader Back End 360 reads per pixel components from Core Back EndFIFO 390 and triangle data from Triangle Memory 410 and computesinterpolated pixel depth values using DPU 450. Interpolated pixel depthvalues are used by Clip 455 to clip pixels using near and far clippingplanes and optionally clamp clipped pixel values to avoid discarding apartially clipped pixel.

In step 522, Shader Back End 360 determines if “depth replace” isenabled according to the codewords and, if so, in step 548 Multiplexor465 selects PCU2 430 calculated displaced pixel depth values as finalpixel depth values to be output from Shader Back End 360 to Combiners370. In step 524, Combiners 370 uses the codewords to determine whetherthis is the last pass of the data through Shader 255 and, if so, in step526 Combiners 370 input final pixel depth values into Depth FIFO 475that are later output to Raster Analyzer 265. In step 528, RasterAnalyzer 265 receives final pixel depth values from Combiners 370 andperforms near and far plane clipping with optional clamping. RasterAnalyzer 265 optionally reads pixel depth values stored in Local Memory240 corresponding the pixel (x, y) locations for final pixel depthvalues. Raster Analyzer 265 optionally performs a depth test functionusing read pixel depth values and final pixel depth values as specifiedby the codewords and generates a pass or fail result. If the depth testpasses, final depth is written back to Local Memory via MemoryController. If the depth test fails the final depth is discarded. Inthis example, final depth is displaced pixel depth that was generatedusing interpolated pixel depth values computed from the coordinates ofthe geometry primitive displaced by normal vectors and map datarepresenting depth displacements.

In an alternate embodiment, final pixel depth values are calculatedusing pixel programs that instruct Shader 255 to process the data inmore than three passes. In the preceeding examples several sources areinput to PCU1 420 and PCU2 430 to generate final pixel depth, including,but not limited to interpolated pixel depth, data processed by theProgrammable Computation Units in the pipeline (such as pixel normalvectors), data read from Local Memory 240, data read from or Host Memory112, data processed by DPU 450 in the pipeline, data stored in Core BackEnd FIFO 390, and data stored in Quad Loop Back 356. Furthermore, depthvalues include interpolated pixel depth, indices used to read pixeldepth, indices used to read vertex depth, and depth components, e.g.,derivatives, differences, normal vectors, etc.

The invention has been described above with reference to specificembodiments. It will, however, be evident that various modifications andchanges may be made thereto without departing from the broader spiritand scope of the invention as set forth in the appended claims. Theforegoing description and drawings are, accordingly, to be regarded inan illustrative rather than a restrictive sense. The listing of steps inmethod claims do not imply performing the steps in any particular order,unless explicitly stated in the claim. Within the claims, elementlettering (e.g., “a)”, “b)”, “i)”, “ii)”, etc.) does not indicate myspecific order for carrying out steps or other operations; the letteringis included to simplify referring to those elements.

1. A method of calculating a pixel depth value, comprising: a)selecting, under control of a pixel program, one or more inputs from aplurality of sources; b) computing a calculated pixel depth value in aprogrammable fragment processing pipeline, under control of the pixelprogram, using the selected one or more inputs, the calculated pixeldepth value associated with a geometry primitive pixel and independentof the coordinates of the geometry primitive; c) writing the calculatedpixel depth value to a local register file; d) computing a firstinterpolated pixel depth value associated with the geometry primitivepixel and dependent on the coordinates of the geometry primitive; and e)computing, under control of the pixel program, a second interpolatedpixel depth value that is the same as the first interpolated pixel depthvalue.
 2. The method of claim 1, further comprising the step of readinga value from the local register file, the read value being one of theone or more inputs from the plurality of sources.
 3. The method of claim1, further comprising the step of reading a value from a graphicsmemory, the read value being one of the one or more inputs from theplurality of sources.
 4. The method of claim 1, wherein one of the oneor more inputs from the plurality of sources is the result of aprogrammable computation in the programmable fragment processingpipeline.
 5. The method of claim 1, further comprising the step ofreading a value from a host memory, the read value being one of the oneor more inputs from the plurality of sources.
 6. The method of claim 1,further comprising the step of choosing, under control of the pixelprogram, between the calculated pixel depth value and the firstinterpolated pixel depth value to generate a final pixel depth value. 7.A method of calculating a pixel depth value, comprising: a) selecting,under control of a pixel program, one or more inputs from a plurality ofsources; b) computing a calculated pixel depth value in a programmablefragment processing pipeline, under control of the pixel program, usingthe selected one or more inputs, the calculated pixel depth valueassociated with a geometry primitive pixel and independent of thecoordinates of the geometry primitive; c) writing the calculated pixeldepth value to a local register file; d) computing a first interpolatedpixel depth value associated with the geometry primitive pixel anddependent on the coordinates of the geometry primitive; e) choosing,under control of the program, between the calculated pixel depth valueand the first interpolated pixel depth value to generate a final pixeldepth value; and f) clipping a pixel associated with the firstinterpolated pixel depth value and conditionally discarding the pixelbased on a near clipping plane and a far clipping plane.
 8. The methodof claim 7, further comprising the step of clipping a pixel associatedwith the final pixel depth value based on the near clipping plane andthe far plane using a second clipping unit.
 9. The method of claim 8,further comprising the step of clamping the final pixel depth value. 10.The method of claim 7, further comprising the step of clamping the firstinterpolated pixel depth value.
 11. A programmable fragment processingpipeline comprising: a) a local register file; and b) a programmablecomputation unit configured to select, under control of a pixel program,one or more inputs from a plurality of sources, the programmablecomputation unit comprising: i) one or more arithmetic units configuredto compute, under control of the pixel program, a calculated pixel depthvalue using the one or more selected inputs, and an interpolated pixeldepth value, wherein the calculated pixel depth value is associated witha geometry primitive pixel and independent of the coordinates of thegeometry primitive, and the interpolated pixel depth value is associatedwith a geometry primitive pixel and dependent on the coordinates of thegeometry primitive; and ii) a write interface configured to write thecalculated pixel depth value to the local register file; c) a depthprocessing unit configured to select, under control of the pixelprogram, one or more inputs for a pixel depth calculation, the depthprocessing unit comprising one or more arithmetic units configured tocompute, under control of the pixel program, a first interpolated pixeldepth value using the one or more selected inputs, the firstinterpolated pixel depth value associated with the geometry primitivepixel and dependent on the coordinates of the geometry primitive; d) aselector configured to select, under control of the program, between thecalculated pixel depth value and the interpolated pixel depth value togenerate a final pixel depth value; and e) a second programmablecomputation unit configured to select, under control of a pixel programone or more inputs for a pixel depth calculation, the secondprogrammable computation unit comprising: i) one or more arithmeticunits configured to compute, under control of the pixel program, aninterpolated pixel depth value using the one or more selected inputs,the computed interpolated pixel depth value being the same as theinterpolated pixel depth computed by the depth processing unit; and ii)a write interface configured to write the calculated pixel depth valueto the local storage resource.
 12. The programmable fragment processingpipeline of claim 11, further comprising a read interface configured toread a value from the local register file, the local register file beingone of the plurality of sources.
 13. The programmable fragmentprocessing pipeline of claim 12, further comprising a second readinterface configured to read a value from a graphics memory, thegraphics memory being one of the plurality of sources.
 14. Theprogrammable fragment processing pipeline of claim 12, furthercomprising a second read interface configured to read a value from ahost memory, the host memory being one of the plurality of sources. 15.The programmable fragment processing pipeline of claim 11, wherein thesecond programmable computation unit in the programmable fragmentprocessing pipeline is one of the plurality of sources.
 16. Aprogrammable fragment processing pipeline comprising: a) a localregister file; and b) a programmable computation unit configured toselect, under control of a pixel program, one or more inputs from aplurality of sources, the programmable computation unit comprising: i)one or more arithmetic units configured to compute, under control of thepixel program, a calculated pixel depth value using the one or moreselected inputs, and an interpolated pixel depth value, wherein thecalculated pixel depth value is associated with a geometry pixel andindependent of the coordinates of the geometry primitive, and theinterpolated pixel depth value is associated with a geometry primitivepixel and dependent on the coordinates of the geometry primitive; andii) a write interface configured to write the calculated pixel depthvalue to the local register file; c) a depth processing unit configuredto select, under control of the pixel program, one or more inputs for apixel depth calculation, the depth processing unit comprising one ormore arithmetic units configured to compute, under control of the pixelprogram, a first interpolated pixel depth value using the one or moreselected inputs, the first interpolated pixel depth value associatedwith the geometry primitive pixel and dependent on the coordinates ofthe geometry primitive; d) a selector configured to select, undercontrol of the program, between the calculated pixel depth value and theinterpolated pixel depth value to generate a final pixel depth value;and e) first clipping unit configured to clip a pixel associated withthe interpolated pixel depth value and conditionally discard the pixelassociated with the interpolated pixel depth value based on a nearclipping plane and a far clipping plane.
 17. The programmable fragmentprocessing pipeline of claim 16, further comprising a second clippingunit configured to clip a pixel associated with the final pixel depthvalue and conditionally discard the pixel associated with the finalpixel depth value based on the near clipping plane and the far clippingplane.
 18. The programmable fragment processing pipeline of claim 17,wherein the second clipping unit is programmable to clamp the finalpixel depth value.
 19. The programmable fragment processing pipeline ofclaim 16, wherein the first clipping unit is programmable to clamp theinterpolated pixel depth value.